Choose Concrete for a Project
Whether you are building a driveway, setting fence posts, or placing a foundation for a large building, you need to know a few basic things before choosing the best concrete product for the job you are doing. Here are some steps to help you do succeed in your project.
Contents
Steps
- Learn about the basic design mixes used in concrete production. There are many different types, strengths, and compositions of concrete, each with characteristics that make them perform better for different applications. Here are a few examples:
- Strength is usually determined by compression testing, and is expressed in Pounds per Square Inch (PSI). Normal concrete mixes range from 2500 PSI to 5000 PSI, with engineered design mixes of over 10,000 PSI used in specialized applications. Lower strengths are also available for grouts, but since these are usually used for displacement, block-fill, and filling abandoned utility lines, we won't spend much time on these products. Here are some common uses for different strength concretes. Note that these are not exclusive, as designers and structural engineers may spec different uses in specialized construction projects.
- 2500PSI concrete may be used in driveways, walkways, and even floor slabs on grade. This concrete is usually the cheapest available from batch plants. Used on solidly compacted fill material (subgrade), this concrete performs satisfactorily for these projects, but many professional concrete workers prefer a higher strength product due to warranty concerns, and some building codes may not allow its use for all of these applications.
- 3000PSI concrete in many locations is a standard multipurpose mixture for general use in construction. It is durable, has sufficient cement to give it good finishing characteristics, and can be placed fairly wet without sacrificing quality.
- 3500PSI concrete is used for applications where surface spalling is not acceptable, and significant loading is expected. One example would be paving curbs, where heavy traffic may drive on the surface. Other uses include building footings, bond beams, grade beams, and floor slabs where heavy loads may be moved or stored.
- 4000PSI concrete is used for heavy traffic pavement, heavy use floor slabs like shops and warehouses, and concrete footings designed to support heavy loads.
- 5000PSI and higher concrete mixes are usually used for specialized construction projects where high impact resistances, very low wear rates, or extreme conditions are expected.
- Special mixes. Concrete can be mixed using different types of aggregates that give it different qualities that may make it more suited for the project. Here are examples:
- Pump mixes are especially designed to make placement with a mechanical pump through a high pressure hose possible with a minimal chance of blockages. They can be pumped wetter than other mixes, and may have smaller large aggregates than standard mixes.
- Exposed aggregate mixes are designed so that when the finished grout surface is removed by sandblasting or water washing while the concrete is very green (not cured), a uniform layer of similarly sized aggregate is exposed. River rock, smooth gravel, or graded crushed limestone aggregate are mixed in this mixture at a higher than normal ratio.
- Entrained air mixtures have air added in the production process, so that tiny air bubbles are created in the finished material. This allows more expansion in the concrete without cracking, making it perform better in projects that are exposed to great differences in temperature.
- Flexible concretes are used for large paving projects, and achieve structural strength without large amounts of reinforcing, thereby reducing material and installation costs. These mixes are describe according to their flexible strength, for example, 650, 750, and 850 flex.
- Strength is usually determined by compression testing, and is expressed in Pounds per Square Inch (PSI). Normal concrete mixes range from 2500 PSI to 5000 PSI, with engineered design mixes of over 10,000 PSI used in specialized applications. Lower strengths are also available for grouts, but since these are usually used for displacement, block-fill, and filling abandoned utility lines, we won't spend much time on these products. Here are some common uses for different strength concretes. Note that these are not exclusive, as designers and structural engineers may spec different uses in specialized construction projects.
- Learn about the characteristics of the concrete you will be using. There are several different characteristics you need to understand to make sure your concrete is placed and finished correctly. Here are some of them:
- Slump. This is a measure of how plastic the concrete condition can be when it is placed. Slump is measured by filling a conical metal mold with the concrete, rodding it in stages to ensure it is consolidated thoroughly in the mold. The mold is then carefully removed by lifting it off, and the distance the concrete slumps is measured. If the concrete falls {{safesubst:#invoke:convert|convert}}, it is considered a "{{safesubst:#invoke:convert|convert}} slump". Design mixes usually have a recommended slump range to yield optimal performance, and adding too much water, while increasing the slump and making the concrete easier to pour, can greatly reduce the strength of the finished concrete.
- Fly ash content. Fly ash is an industrial by-product used to supplement the Portland cement in may design mixes. It decreases the production cost of the finished material, and makes repairing spalling and other surface imperfections easier. It also slows the setting speed of the concrete, allowing more time to work the material during the finishing process.
- Chemical admixtures. There are many chemical products that can be added to concrete that enhance certain performance characteristics of concrete. Plasticizers make concrete very fluid to allow placing it in tight form confines, or where the reinforcing makes it difficult to get good fill during placing. Water reducing mixtures decrease the amount of water required to improve its workability without effecting strength, and shrinkage reducing admixtures reduce the need for expansion and construction joints where these are undesirable.
- Understand the different colors available in concrete products. Most people picture concrete as a drab, grey material, but many integrally colored concretes are available, ranging from pure white, to blues, reds, browns, and almost any other color you can imagine. Finished concrete can also be acid-etched, stained, or coated with epoxies or shakes to give unique effects if so desired.
- Look at the many different types of finishes that can be executed on concrete. Plain, smooth finishes are accomplished by floating and troweling the concrete, and by and large, this is the most common method of concrete finishing. Other methods are available, including the following:
- Broom finishing is done, as the name implies, with a broom while the cement paste on the surface is still workable. Brooming is a good option for areas like sidewalks and driveways, where traction is needed.
- Salt finishing is done by sprinkling rock salt on the still workable cement paste surface. This are then floated or troweled down into the surface before they dissolve, and once they have dissolved they leave a pock-marked surface that when properly done gives a unique, distinctive appearance and good traction.
- Exposed aggregate, mentioned earlier, is done by washing or sandblasting the cement paste surface to expose the rock underneath. When done with uniformly colored and size aggregate, this can be a very attractive finish.
- Embossed concrete is done with special templated rollers or grids which are stamped or pushed into the cement paste while it is still workable.
- Talk to local professionals for help determining the concrete you will need for your product. Local Ready Mix concrete suppliers should have sales people or product representatives who can recommend the best material to use for the project you are planning. This is especially useful since specific conditions may make products used in one region completely unsuited for another region.
- Read plans and specifications if you are following an engineered construction plan for your project. A structural engineer with experience in concrete design often produces specifications on the exact design mix needed for good performance.
- Use the above information to decide the type of concrete mix best suited for your project. To make sure the concrete performs well in its application, you also need to do your homework on three other items of consideration in building with concrete.
- Form the concrete correctly, making sure the forms are level or plumb where applicable, and rigidly constructed to avoid deflection or failure. Using strong materials and sufficient bracing is critical to successful concrete placement.
- Make sure the reinforcing material is correctly installed. If you are using rebar, tying and supporting it in the correct position is necessary for it to give the desired strength. Rebar that is not completely encased in the concrete can also corrode, causing damage to the concrete structure itself.
- For on grade slabs, make sure the subgrade is structurally sufficient to support the weight of the concrete and other elements of your building. You may have to remove unstable material and replace it with structural fill to achieve the desired load-bearing threshold your project requires.
Tips
- Experience is the most valuable tool you can have when selecting concrete for a project, so seek help with expertise in concrete construction practices in your area before beginning your project.
Warnings
- Choosing the wrong concrete mixture can result in your finished project failing to meet expectations. This can mean a cracked, spalled driveway, or structural failure in walls, beams, or foundations.
